1. Field of the Invention
The present invention relates to a heat treatment apparatus for subjecting a sheet which must be subjected to heat treatment, to heat treatment and more particularly to a heat development apparatus for use in a recording operation which is performed in a dry system, such as an image recording operation using a dry material such that a wet process is not performed.
2. Description of the Related Art
An image recording apparatus for recording a medical image for use in a digital radiography system, a CT, an MR or the like which uses a heat accumulating fluorescent sheet is known. The foregoing apparatus employs a wet system for obtaining a reproduced image by performing a wet process after an image has been photographed or recorded on a silver-salt photo-graphic photosensitive material.
In recent years, a recording apparatus has attracted attention which employs a dry system in which the wet process is not performed. The foregoing recording apparatus is arranged to use a photosensitive and/or thermo-sensitive recording material or a film made of a heat development photosensitive material (hereinafter called "recording materials"). In the recording apparatus using the dry system, the recording material is irradiated (scanned) with a laser beam in an exposing section so that a latent image is formed. Then, the recording material is, in a heat development section, brought into contact with a heating means so that heat development is performed. Then, the recording material on which an image has been formed is discharged to the outside of the apparatus.
The dry system of the foregoing type is able to form an image in a shorter time as compared with the wet process. Moreover, the problem of a necessity of disposal of waste liquid produced in the wet process can be overcome. Therefore, increase in the demand of the dry system is expected.
The foregoing dry system is usually structured to incorporate the heat development section having a heating means which is a heating drum. An endless belt is wound along the surface of the heating drum for a predetermined angle so that a recording material is held and transferred by the heating drum and the endless belt. Thus, heat development is performed. If the tension of the endless belt becomes non-uniform owing to heat deterioration or the like, uniform contact between the recording material and the heating drum cannot be realized. Thus, irregular development occurs.
Since images for use in the medical field must have a high quality, the recording materials have significantly high sensitivity. If the state of contact with the heating drum encounters slight non-uniformity, the image quality excessively deteriorates.
In the heating means, a decrease in the temperature in an edge portion to which heat is not greatly supplied, in particular, contact between the surface of the heating member and the low-temperature sheet causes a temperature gradient of the heating member to occur in a direction in which the sheet is transferred. Since the foregoing temperature gradient cannot quickly be overcome, differences in the temperature occur in the surface of the sheet or among sheets in a case of a sequential process. Thus, there arises a problem in that the quality of a result of the heat treatment deteriorates.
When the recording material is held between the heating drum and the endless belt, there arises a problem in that buckling of an end portion causes undesirable folding and a crease.